Research and development of a phase shifted distribution feedback semiconductor laser (simply called "a phase shifted-DFB laser hereinafter) in which a diffraction grating having a .lambda./4 shift in the axial center of an active region is formed has been continued because such a phase shifted-DFB laser is used for a light source adapted to a long distance and large capacity communication using an optical fiber for the reason why a stable oscillation can be performed with a single wavelength in the DFB laser even under direct modulation. Advantages of a phase shifted-DFB laser as compared to a DFB laser having a continuous diffraction grating is that a stable single wavelength oscillation in which a sub-mode is sufficiently suppressed can be performed because a threshold gain difference is large between main and sub-modes even at a modulating time, and that a high yield is obtained in being fabricated. For the reasons, development and practical use of the phase shifted-DFB laser are strongly desired in view of an application thereof to an optical system and a fabrication of devices.
A kind of a phase shifted-DFB laser was fabricated for research and development by the inventor. The phase shifted-DFB laser was fabricated in following procedures. That is, a guide layer of n-InGaAsP having a thickness of 0.1 .mu.m, an active layer of non-doped InGaAsP having a thickness of 0.1 .mu.m, anti-melt back layer having a thickness of 0.03 .mu.m, and a cladding layer of p-InP having a thickness of 1 .mu.m were grown on an n-InP substrate of (001) plane orientation having a diffraction grating with a .lambda./4 shift which is parallel to a crystal orientation of [110] plane by liquid phase epitaxy, and a mesa stripe were then formed at a portion corresponding to an active region of a multi-layer semiconductor crystal to be positioned between two parallel grooves each having a depth of 3 .mu.m along [110] orientation. In a window region, no mesa stripe was formed, and a groove was provided with a width equal to the sum of width of the two parallel grooves and the mesa stripe. The two parallel grooves were communicated to the groove at a position where the mesa stripe was terminated. Then, current blocking layers of p-InP and n-InP were grown except for above of the mesa stripe, and a buried layer of p-InP and a contacting layer of p.sup.+ -InGaAsP were grown on a whole surface of the blocking layer and the mesa stripe. Thereafter, a SiO.sub.2 film was grown on the contacting layer except for a portion corresponding to the mesa stripe, and a pair of electrodes were provided on upper and back surfaces of a multi-layer semiconductor as processed above. Finally, a one side facet of the multi-layer semiconductor on which the active region was positioned was coated with an anti-reflection film to complete the phase shifted-DFB laser.
According to the observation of phase shifted-DFB lasers thus fabricated, a yield of 100% is obtained in fabricating the phase shifted-DFB lasers each having a property of a single wavelength oscillation.
However, a high oscillation threshold current is required to be 40 mA much higher than 20 mA which is that of a DFB laser having no window region due to leakage current flowing outside the active layer in the mesa stripe, although the more detailed reason which is analyzed in an experiment by the inventor will be explained later.